1. Field of the Invention
The present invention relates to a method for data encryption and a method for conjunctive keyword search of encrypted data, and in particular, to a method for data encryption and a method for conjunctive keyword search of encrypted data to prevent data of a user from being leaked from an external server when important data of a user are stored in the external server.
2. Description of the Related Art
Recently, in respects to a leakage case of customer information from an enterprising database, a security problem with information stored in an external storage space has been an issue. Access control or key management techniques that have been mainly used for protecting information in a database, etc., is an effective method for stopping external hackers but does not basically prevent an owner of an external storage space from reading, leaking, and maliciously using the stored data. In order to safely store information, a method for encrypting and storing data is first considered. However, a general method used for data search is impossible to search the encrypted data, a search method considering this situation is needed.
The encrypted data search, which is a technology of searching desired data without decrypting the encrypted data, was first attempted by Song, et al. (IEEE Security and Privacy Symposium 2000). Most of methods mainly perform a search for one keyword.
In addition to the basic search, when considering conjunctive keyword search for several keywords, the easiest method to produce search results is to perform one keyword search several times. However, it is important not to expose the information on each keyword while performing the conjunctive keyword search for several keywords in respects to the privacy issue of the conjunctive keyword search.
For example, when searching a (A∩B) document including both of A and B, the user does not expose either information of other encrypted data including only A in addition to encrypted data from the search results nor the fact that the document is searched by a keyword to be ‘A’. This is also similarly applied to ‘B’ herein. In this case, an applicable method is to store the conjunction results for all the keywords as indexes for the document. However, the method should store mC1+mC2+ . . . +mCr indexes when the number of keywords for each data is m and the conjunction for keywords of r or less is stored. In worst case, if r=m, since approximately 2m indexes should be stored, there is a problem in that the storage space is exponentially increased.
To solve the above problem, the conjunctive keyword method was proposed by Golle, et al. (ACNS 2004), while several studies have been progressed. Most of the conjunctive keyword search has mainly used a method based on pairing operation (it has been known that one-time pairing operation is similar to a speed of performing exponentiation operation about 6 to 20 times) but require a lot of operations, as a result, the method has a problem in view of efficiency and a limitation in a practical use.